Understanding cement grout diffusion in rock fractures is crucial for rock engineering, yet grouting faces significant challenges due to fracture network heterogeneity and grout's complex non-Newtonian rheology. This study critically reviews recent theoretical, experimental, and numerical advancements to comprehensively understand cement grout diffusion mechanisms within rock fractures. It begins by discussing theoretical foundations, encompassing both continuum and particulate views in single fractures, while also highlighting limitations in extending these simplified concepts to fracture networks and defining robust stop criteria. Subsequently, the article details developments in experiments, including novel apparatus and advanced monitoring techniques. These enable controlled observation of grout diffusion in artificial or simulated fractures, providing crucial insights into the impact of fracture complexities (e.g., fracture roughness, two-phase flow) on grout patterns and sealing efficiency. These laboratory tests also inform the development of practical stop criteria by revealing actual grout behaviour under various conditions. Complementary numerical methods offer a distinct advantage by providing dynamic, continuous solutions for complex fracture networks that are otherwise intractable. Collectively, these diverse approaches bridge critical knowledge gaps, from fundamental principles to real-world complexities, and facilitate cross-scale validation. The review concludes by identifying persistent challenges, such as integrating multi-scale descriptions and simulating true field complexities, and outlines future research directions to understand grout diffusion mechanisms.